Project Summary Mast cells (MCs) are effector cells in asthma and their activation causes secretion of cysteinyl leukotrienes (cys-LTs) and prostaglandins (PGs). MCs not only secrete these mediators, but they also possess receptors for them, to perceive their signals. Cys-LTs are potent bronchoconstrictors, powerful inducers of vascular leakage, potentiators of airway hyper-responsiveness and play an important role in asthma and other inflammatory disorders. Cys-LTs mainly act through two G-protein-coupled receptors (GPCR), CysLT1R and CysLT2R. Another GPCR, GPR17 is activated by LTD4. Both CysLT2R and GPR17 negatively regulate CysLT1R function. LTE4, the most abundant and stable of the cys-LTs, is a weak, partial agonist for the CysLT1R and CysLT2R. However, LTE4 induces unique responses in vivo that cannot be recapitulated by LTC4 or LTD4. In MC, LTE4 is more potent than LTD4 and relays signals through peroxisome proliferator activating receptor (PPAR)-? and P2Y12R. Recently, GPR99 was identified as another CysLTR with a preference for LTE4. Understanding how all these cys-LT receptors (CysLTR) interact with each other in response to multiple ligands is critical, especially considering their role in airway physiology. Further, cys-LTs together with PGE2 synergistically potentiate calcium flux, c- Fos, COX-2, PGD2 and Macrophage Inflammatory Protein-1? (MIP-1?; CCL4)) generation in MCs. Interestingly, LTD4-PGE2 synergism is blocked only by combined treatment of CysLT1R antagonist (MK571/ singulair) and EP3 antagonist (L-798), suggesting the need for a combination of CysLT1R antagonists and EP3R antagonists to treat inflammation in asthma. LTD4+PGE2 synergism also potentiates pulmonary inflammation in der f sensitized mice (recruitment of immune cells, goblet cell metaplasia, up-regulation of inflammatory transcripts). Similar to LTD4, LTE4 synergizes with PGE2 but it differs from LTD4 via signals involving PPAR?. Based on these observations, we hypothesize that cys- LTs induce complex interactions between cys-LT-responsive receptors to profoundly influence the downstream signaling by switching anti-inflammatory PGE2 signaling to pro-inflammatory, upregulating COX-2 and PGD2 production in MC impacting Th2 inflammation and asthma. We will test this hypothesis in the following specific aims: 1) To determine the interplay between the known (CysLT1R and CysLT2R) and putative (GPR99, P2Y12R and PPAR?) CysLTRs in response to cys-LTs, influencing MC function, 2) To uncover the mechanism by which cys-LT-PGE2 synergism induces PGD2 production and MC activation and 3) To determine the physiological significance of cys-LT+PGE2 interactions and MC in pulmonary inflammation in vivo. We will analyze pathologic, physiologic, and immunologic signatures of the immune response and evaluate the contribution of MCs. These studies will carry substantial pathogenic and therapeutic implications for asthma and allergic diseases as well as provide the basis for development and translation of future therapeutic molecules that regulate inflammation.